Today, the signal design of current-measuring or current-sensing semiconductor switches is usually based on a current-sensing behavior in the normal operation, in which a sense current proportional to a load current IL=Isense·KILIS is output, and a defined signal output in case of an occurring fault. In this case, a defined current or a defined voltage, for example an infinitesimal current or an infinitesimal voltage (value 0) or a maximum current or a maximum voltage, is output in case of a switch-off attributable to a fault state. In principle, this signal design permits a differentiation of the operating cases of load interruption (open load), in which the sense current or the sense voltage take on a sufficiently small value, normal operation, in which the sense current or the sense voltage is in a predetermined and monitored range, and the fault case, in which a defined sense current or a defined sense voltage is output, as explained above. This signal design, however, is optimized for semiconductor switches without open load diagnosis in the switched-on state (ON). FIG. 12 shows the presently common, principle designs of such a load signal.
FIG. 12 shows two schematic illustrations of an exemplary signal design of a voltage course VIS of a sense pin of a semiconductor switch as a function of a load current IL flowing through an external component, wherein a case in which a defined voltage is output is illustrated in FIG. 12a, and a defined current in the fault case in the case illustrated in FIG. 12b. In FIGS. 12a and 12b, a course of a VIS-IL course each is schematically shown for a measuring resistor with the value RIS—1 850-1 and 860-1 attached to the sense pin of the semiconductor switch and for an electrical resistor RIS—2 850-2 and 860-2 smaller as compared to RIS—1. Here, the voltage VIS at the sense pin rises, starting from an infinitesimal load current (IL=0), in substantially linear manner with the load current IL for all cases illustrated in FIG. 12, until the load current reaches and exceeds a predetermined maximum value IL—FAULT. From the load current IL—FAULT on, a constant and predetermined voltage VIS—max is provided at the sense pin of the semiconductor switch in the waveform illustrated in FIG. 12a, so that the constant voltage VIS—max is present at the sense pin independently of the electrical resistor attached to the sense pin. In the waveform illustrated in FIG. 12b, upon reaching and exceeding the limit IL—FAULT by the load current IL, a constant current, which leads to a corresponding voltage signal VIS at the sense pin of the semiconductor switch, depending on present electrical resistance, is output at the sense pin.
Moreover, FIG. 12 schematically shows three load current regions, namely an open load load current region 870-2, a normal operation load current region 870-4, and a fault case load current region 870-6. Due to the jump that sets in when exceeding the load current threshold IL—FAULT of the VIS-IL characteristic curves 850-1, 850-2, 860-1, 860-2, only the error case load current region 870-6 out of these three load current regions is uniquely identifiable. Particularly in the range of small load currents, a unique differentiation of the load current regions open load 870-2 and the normal operation load current region 870-4 hardly is possible, since the voltage VIS present at the sense pin of the semiconductor switch depends on the resistor attached to this pin on the one hand, and the voltages VIS resulting in these load current regions frequently lie in the range of zero point voltages or also noise amplitudes of typical semiconducting devices in typical applications, for example in the motor vehicle field.
In other words, this signal design is not or not optimally applicable to semiconductor circuits with an open load diagnosis in the switched-on state, as for example described in the non-published German patent application DE 10 2005 030 150.9.